I may be a fringe dweller but I read every thread on Everything Diesel. However, every so often I post a question. This time, tuned pipes or "magic" mufflers on diesels as against the same motor as left as a 2 stroke, on a model meant purely for speed. Will a diesel on pipe perform as against a 2 stroke on pipe? Lashed out and bought myself a 2nd ARF after 50 years of scratch building. Shame, shame, shame. This one's a MODEL TECH Mach Racer delta flying wing, weighing 3 1/2 lbs, for a .40 to .46, based loosely on the American Diamond Dust model. (goes like a cut snake) I have a spare Vmax .46 (nib), plus a new dieselized head for same. My personal opinion, not possible as I'll be reducing the size of the venturi when converting to diesel but for speed, I'll need to have a large bore as possible. Stick with the glo for this model? Pity, as I enjoy my diesels. Your thoughts, please. Glen

Glen,
All infernal combustion engines benefit from a tuned exhaust, but as you know our model diesels are more tolerant to exhaust missmatch.
Will you get the same jump when it comes on pipe as a diesel as compaired to glow ???? that all depends on the individual motor, its bore stroke ratio and its porting. From my reading a diesel in general responds to a shorter tuned pipe as apposed to the same motor on glow but again there all individuals.
Stewart

Hi, Stewart. The thought of using a diesel in a speed machine never occurred to me til my son's eyed opened wide when he noticed the conversion diesel head in with the motor and came up with the suggestion. Never ever considered using a tuned pipe on a diesel before this suggestion. The more I think about the pros and cons, not so practical. Converting to diesel usually means "propping up" whereas for speed I need to reduce the prop size and increasing the pitch. This together with reducing the carby bore size. But then again, nothing ventured, nothing gained. I might just give it a try. I may learn something new. I've tuned pipes on glo so know the concept and principal. Glen

A diesel will run faster with bigger bore up to a point, just like a glow engine will. I think the "up to a point" may be smaller for diesel than glow, but don't know. How much power an engine puts out is directly related to rate of airflow through the engine. I'd fly the airplane with about 9 x 8 on glow, then try it as a diesel on the same prop, or maybe a 10 x 8. Don't know about pipes.

Tuned pipes and mufflers do work on our diesels. The results will depend on how "peaky" the engine pipe combo is. If the combo is set up for very high power, I think it will be very hard to throttle well. The thing to consider is that a pipe potentially accomplishes two things. First it helps scavenge the cylinder. This makes sure all of the spent gases are removed and fresh charge is drawn in from the crankcase. Second it can help pack the cylinder after the transfers close by pressurizing the clyinder just before the exhaust port closes. The first is beneficial to any engine. The second might not work too well in our diesels because you must reduce compression settings to get proper timing. This potentially reduces power output, and causes issues when the engine is "off the pipe".

A broadly tuned system(engine and pipe) should work just fine with a diesel. I say system because both play a part.

One does not need to prop up with diesel. This really depends on the engine. Some don't make any more torque as diesels, and some rev just as high as glow on diesel.

The venturi for diesel will generally be quite a bit smaller at the same power output. I've seen as much as 50% less area is acceptable. For instance, an F2C team race diesel runs a venturi about 4.5mm diameter, yet makes around 1HP.

I have not tried it but I understand that you can't effectively use a tuned pipe on a diesel because the charge that would get reflected back in is just spent gasses because diesel combustion is more complete.

Yes/No?

George

One stunt that works is a multi pitch prop at the suggestion of Bob Davis I went from a 12x6 to 12/6/10
it was a vmar hot rod high wing not really designed for speed but is was faster with the prop 12/6/10 once airborne it comes into play it was on an irvine 53 conversion martin

The reason people put larger props when moving up to diesel is the increase in power.

You should beable to increase the pitch instead of the length to better use the extra power.

Dare I say that most who run diesel want more power but not more speed
that is why they go with the longer prop.

I have mixed feelings on the tuned exhaust.
I don't believe you would get as much gain as with non-diesel engines,
because of the more effecent combustion.
But you start off with more anyway.

Just my 2 cents worth.

No, a tuned pipe pulls exhaust out of the cylinder, then continues to pull fresh mix into the cylinder and into the pipe. The reflected wave then packs the fresh charge from the pipe back into the cylinder just before it closes. This is, in theory, the way to make the most power. Whether or not this actually occurs depends on many factors.

Time and time again claims are made that diesels make more power. This is NOT true of every engine. The original post is about going fast. Every engine diesel or glow makes peak power way up in the RPM band, not lumbering along at 7-8kRPM.

I always point out the MVVS 2.5cc. From the factory both are rated at similar peak power and RPM. They are rated at .7HP at 26-27kRPM. The diesel does just fine swinging a 9x4 at 12kRPM, but it's not making anything near .7HP. Tuning for speed is a complex matter and one might find that top speed is with a prop at 18 or 20kRPM, but you won't have a chance at going very fast at 12KRPM regardless of the prop.

I have to agree with what Greg said re power and rpm.
Further to that , why do we swing bigger props? well as we all know diesel exhaust is cooler than a glow exhaust , why ? well diesels generally have completed combustion by the time the exhaust opens wereas a glow is still trying to burn the fuel so flame is emitted out the exhaust- dont beleive me , try running both at night with open exhausts.
Its the more complete combustion and the higher energy fuel that gives diesels both the better fuel ecconomy and the higher torque at low rpms, hence the ability to swing the bigger props better than a glow.
Now will the diesel conversion on a pipe go as well as on glow, it should but as Greg said it will require a similar sized prop to do so.
only benefits of running a diesel on pipe are better fuel ecconomy (we still have higher energy fuel) and the COOL factor to make the diesel nay sayers stand back
Stewart

What your really doing here is compareing apple and oranges.

Diesel conversions are set up to use glow fuel so their port timing is for glow fuel.
Engines that are made to run on diesel have their timing to benifit running.

A tuned pipe for glow shouldn't do well on diesel ... Different port timing.

Some where some body has already been here and done this.
Not just speckulated about it. Where is DDD when you need him?

Stewart you guys do not do too bad running the PAWs in some sort of a race thing as is, and Happy New year ( or course to all.) Just got back from dinner out with a few good friends one of which had her mum over from Ireland , in the course of conversation said her brother moved to three springs down there about 40 years ago which I gather in the western part ,raises and harvests clover, guess used as cattle feed
Now back to the
fun stuff after my crash program to finish up the mini tele, was planing the maiden flight New Years day
looks like the weather is not going to cooperate martin
my next issue is trying to find a TT10GP it is the same head as the OS but has a front carb with the needle
angled back TT decided to take the engine out of production too bad

I have run a MACs tuned pipe on an MVVS 2.5. I ran it with an 8x4 prop, but this was when I was a beginner in RC, 12 or more years ago. I do remember it ran fine and seemed to come up on pipe as would be expected. This engine however now has a blown little end in the rod, but I think that is related to running it once every few years and not cleaning the lubrications holes in the rod prior to doing so.

I remember an old RCM article(I should still have it somewhere) about a ducted fan running a Rossi .80(?) on a pipe and the author said it was fine as diesel and made more power than the glow setup. They took an existing airplane and dieselized it.

Thanks for all your thoughts and input. As Greg said, the original thread was for speed only, not for 3D or flying under a limbo pole. My intent is for top rpm with the correct prop combo. Please dont get the impression I'm a rev head intent on achieving destruction of my motor. I treat all my engines as I would my "crown jewels". The delta model to be used for, shall I say, this experiment, only has a 20mm (just over 3/4 inch) depth at the wing root chord, tips tapering to 12mm (1/2 inch)= for speed only. My son will be flying this model; will be too fast for me "on the pipe". I'm too old but not yet senile (well, I don't think so. I still recognize the bloke looking back at me in the mirror). Pleased me to read in the posts where tuned pipes do actually work on diesels. The reference to an old RCM about ducted fans and diesels really drew my attention. Another field I'm involved with (fans and Rossi's and K & B 7.5). I still get a great kick out of the "newbies" at the strip when they see me start my diesel motors without a glo driver or an electric starter, just using the old digit. The gasps, dropping of jaws. Priceless. But will definately not use that same digit on the 9" prop on the dieselized Vmax 46. Electric starter here. Yes, I know about hydraulic lock. Looks like I take the plunge and dieselize. Glen